Gabbro and diorite of southeast Alaska

Unit symbol: Kgb
Age range Cretaceous (145 to 66 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Primarily gabbro, hornblende gabbro, hornblendite, leucogabbro, and subordinate norite, syenite, and pyroxenite on Prince of Wales Island (Eberlein and others, 1983), Kuiu Island (Brew and others, 1984), and Chichagof Island (also described as the layered gabbro near the head of Tenakee Inlet by Loney and others[1975]), where it is associated with Early Cretaceous granodiorite and diorite (units Keg and KJse) (Loney and others, 1975; Johnson and Karl, 1985). Also includes hornblende diorite and gabbro in the Haines area that grades laterally into granodiorite and quartz diorite. Potassium-argon ages are Late Cretaceous (Redman and others, 1984). Hornblende and biotite diorite of the Jualin Diorite (Knopf, 1911) along east shore of Lynn Canal in the northern Juneau quadrangle has produced a hornfels aureole in adjacent metasedimentary and metavolcanic rocks. Gehrels (2000) reported a U/Pb zircon age of 105 Ma on this pluton. At an unusual occurrence in the Craig quadrangle, epidote replaces the gabbro in irregular masses up to several meters in diameter or as veinlets cutting the rock (Herreid and others, 1978). In thin section, the replacement origin of the epidote is clearly shown by gradational embayed contacts with gabbro. The gabbro, which is a small border phase of Cretaceous granodiorite, contains medium-sized subhedral crystals of sericitized labradorite and subordinate hornblende, pyroxene, ilmenite, sphene, and apatite (Herreid and others, 1978)

Source map information

Source map Brew, D.A., and Ford, A.B., 1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, pamphlet 23 p., 2 sheets, scale 1:250,000.
Symbol Kmdi
Unit name Hornblende diorite
Description GRAVINA BELT: INTRUSIVE ROCKS OF ADMIRALTY-REVILLAGIGEDO PLUTONIC BELT AND ASSOCIATED MIGMATITE: Hornblende diorite, quartz diorite, and minor gabbro; medium to very coarse grained; color index 15 to 50; weathers light to dark green; highly altered to epidote and chlorite rich rock. Crops out southeast of Berners Bay, south of Herbert River, and on Douglas and Lincoln Islands. Differs from other Cretaceous granitic rocks in the high degree of alteration and (or) metamorphism. Body on Lincoln Island described by Barker (1957) as: "Gabbro-- Sericitized hornblende uralite gabbro."
Lithology Igneous

Correlated geologic units

Label Kdqd
Description Mafic plutonic rocks, mostly diorite and quartz diorite with subordinate tonalite and gabbro 85-110 Ma. Southeast Alaska
Geologic age Aptian to Santonian
Geologic setting Intrusive, tonalite-granodiorite
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Major
Quartz-diorite < Dioritic < Plutonic < Igneous Major
Tonalite < Granitic < Plutonic < Igneous Minor
Gabbro < Gabbroic < Plutonic < Igneous Incidental